It is known that an underwater vessel (i.e., a submarine) generates sound, which is generally referred to as passive sound, as it travels through the water. The passive sound is generated by a variety of sources, including, but not limited to, sound generated by a submarine propulsion system, sound generated by a submarine propeller, and sound generated by a submarine electrical power generator. It is known that submarine designers attempt to reduce these and other passive sound sources in order to make a submarine difficult to detect by acoustic means, therefore remaining as covert as possible.
Some anti-submarine warfare (ASW) sonar systems attempt to detect the passive underwater sound generated by an enemy submarine. Some other ASW sonar systems attempt to both detect the passive sound and also to localize and/or track the enemy submarine. Localization is used to identify a position of the enemy submarine in azimuth, and/or in range, and/or in depth.
Passive ASW sonar systems attempt to detect, localize, and/or track the enemy submarine using the received passive sound only. The passive sonar system can remain covert and undetectable by the enemy submarine. Some known passive sonar systems use beam-forming techniques to generate receive beams. The receive beams can be steered azimuthally to detect, localize, and/or track the enemy submarine in azimuth. The receive beam can also be steered to vertical angles.
Even at relatively short ranges, localization in depth and range is not generally possible when receiving only passive sound and depending upon a pointing direction of receive beams (directed to a vertical beam steer angle). This is because for any receive beam and associated vertical beam that points toward an enemy submarine, the enemy submarine can be positioned at an essentially infinite number of depths and ranges along the vertical beam steer angle.
At longer ranges, localization of the enemy submarine in range and depth is made even more difficult by a variety of factors, including but not limited to, a tendency of the passive sound generated by the enemy submarine to bend (i.e. refract), primarily in a vertical direction, as the sound propagates through the water. Therefore, the vertical angle of arrival at which the greatest amount of sound arrives at the sonar system, which is related to a particular receive vertical beam angle, does not necessarily point in the direction of the enemy submarine.
However, it has been shown that vertical angles of arrival may be used to determine range and depth. Conventional techniques to determine vertical angles of arrival are generally applied to single path vertical angles of arrival at an array. With a single path, the vertical angle of arrival may be determined using a variety of techniques including monopulse or multibeam interpolation techniques.